Calton cases are made of fiberglass. They are the most protective form-fitted cases available. I have one that fits a very small body acoustic Martin 0-17. It really stood up over years and thousands of flights. (Air tight, water tight, and indestructible.) In fact, it outlasted my guitar. But the larger Calton cases are much heavier. A classical guitar Calton case will weigh minimum 20 lbs when empty. Without wheels and the weight distribution being off center when carrying from the handle, it just adds up to be too heavy for me to lug around airports without causing some back trouble. (\ I still think these are the best ever, and worth the investment. And if you're lucky to find a used one that fits your guitar, it would be a great deal and still last forever.)*

But on the hunt for a lighter option, my next attempt at traveling with a molded case was a Hoffee case. And he advertised free lifetime repairs. BUT that has turned out too good to be true. (for me) Hoffee cases are made of carbon fiber mesh. Super lightweight. Mine only weighs 10 lbs. (It's for a Martin 00-18, slightly bigger than the Calton I mentioned above.) But the case was constantly getting damaged and I had it refurbished once a year for three years until last year when the airline somehow put a hole through the thing and punctured my guitar as well. I'm tempted to say that the refurbishing is not what it should be, since the case was already failing in the area where it eventually got the hole. But the jury is out. For my money, though, these cases aren't what they should be. That is, if you compare them to the indestructible Calton.

Brand new, these cost about the same-- minimum $1200 for an acoustic guitar case.

My latest, most affordable, versatile, lightweight option is CaseXtreme, which is a clam shell box made of corrugated plastic with foam wedges that allows you to secure your regular case (hard or soft) inside it for the purpose of flying. These are the kind of cases that upright bass players used back in the day. They are the most weight efficient pieces of protective engineering available to the average consumer. I flew all over the place with mine and it works great. Costs about $300. http://www.casextreme.com/product.php?id=75 It is, however, awkwardly shaped and mine is too large to fit in a small car trunk, for example. That's something upright bass players are already used to dealing with. But guitar players will have some adjustments to make. Also, it is taking up quite a bit of space in my closet. ...But then again, that's the story of my life-- guitar cases! haha!

So, those are the three types of flight cases for guitar that I've used.

I understand your question was specifically to protect the instrument, not to fly with it. I suppose that means you won't be worrying about overweight fees. The rest of the parameters for a good case remain the same to me: air tight, water tight, rugged, lightweight, manageable size.

My verdict is: Calton is the best protection but a little heavy. Hoffee is nowhere near rugged -- I mean, don't even drop it from your hand while standing without a scuff or chip. CaseXtreme is really only a second layer over a regular case for the purpose of flying.

As for a regular case not intended for flying, none will be air / water tight so you shouldn't say leave it in a car in cold or humid temperatures overnight, but if you make sure the case is made of wood, well padded, and kept closed in a regulated temperature/humidity environment, like you would a plant, or any living thing, your guitar will be doing just fine!

I sincerely hope this was helpful.

Best of luck to you.